I've got a 25 year old house, whose light switches I'm converting over
to a powerline protocol called INSTEON (see www.smarthome.com; for
purposes of this question, it's very similar to the old X10 protocol).

Two of the bedrooms are on one circuit breaker (the whole rooms:
lights, outlets, the whole shebang). Each room has a single switch
controlling the lights in that room. I pulled a light switch out this
evening, planning to replace it, when I saw there was no neutral in the
junction box (the INSTEON switches require both hot and neutral). At
first I expected to find neutral in the wall behind the box (I was
guessing they had, for some reason, routed hot through the box and to
the switch while leaving neutral unbroken), but then realized that what
went in the box was a single piece of three-conductor wire, with black
used for hot, white for switched, and ground not connected.

So... before I start digging the junction box out, what are my odds of
finding neutral somewhere in the vicinity? More generally, I'm having a
hard time imagining why someone would wire the ceiling light fixture and
not have power and neutral come from the same place! So, before I get
myself in trouble, how did they likely wire it?

Thanks in advance,
--
As we enjoy great advantages from the inventions of others, we should
be glad of an opportunity to serve others by any invention of ours;
and this we should do freely and generously. (Benjamin Franklin)